One form of a modulation employed for digital data transmission is Gaussian Minimum Shift Keying (GMSK). In the transmitter a premodulation Gaussian low pass filter (LPF) is used. The Gaussian LPF inherently creates inter-symbol-interferences (ISI) in the modulated signal. Thus, unlike a minimum shift keying (MSK) modulated signal, the phase of the GMSK modulated signal does not fully change to +.pi./2 in response to each bit. Instead, the actual phase change for each bit is less than or equal to .pi./2 and is a function of bit pattern and the BT product of the Gaussian LPF, where B is the band width of the LPF and T is the bit time interval.
Often frequency drifts in the carrier frequency are introduced in the transmitted modulated signal by hardware imperfections in the carrier frequency generation circuitry. Also, variations in vehicle speed can cause additional frequency drifts in the received signal. Automatic frequency control (AFC) systems have been used to track slow frequency drifts in the modulated signal. Typically, AFC systems rely on an error signal generated in a feedback control loop, the error signal ideally representing frequency tracking error.
One known prior art AFC system employs a frequency compensator and error detector with a loop filter in the feedback loop of the frequency compensator and error detector. The error detector estimates the frequency error or offset of the frequency compensated received signal. The loop filter accumulates or integrates the output of the error detector to smooth out noise for the frequency compensator. The frequency compensator compensates the frequency offset of the received signal by an amount proportional to the output of the loop filter.
AFC systems have been developed for various modulation schemes such as binary phase shift keying (BPSK), quadrative-phase shift keying (QPSK) and multiple frequency shift keying (MFSK). The different systems are implemented predominantly in the error detector block depending upon the modulation format of the received signal. Unfortunately, none of these known systems can be used with a GMSK-modulated signal because of the inherent ISI of the GMSK-modulated signal. For example, if an error detector of an AFC system designed for use with a QPSK-modulated signal is used in conjunction with a GMSK-modulated signal, the output of the error detector would be very noisy resulting in a poor AFC performance. This is because the data modulation effects cannot be removed from the error detector output signal due to the ISI affecting the signal phase diagram.
Thus, it is desirable to provide an AFC system for a GMSK-modulated signal in which a reliable error signal is generated thereby improving the overall performance of the AFC system.
It is also desirable to provide a reliable AFC system using inexpensive hardware.